*{
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}
ol,li{
    list-style: none;
}
body{
    overflow: hidden;
}
.page{
    display: flex;
    flex-wrap: wrap;
}
section{
    border: 1px solid purple;
    width: 50vw;
    height: 50vh;
}

#app1{

}
#app1.output{

}
#app1.actions{

}
#app2 > #tab-bar{
    display: flex;
}
#app2 > #tab-bar > li{
    border: 1px solid green;
    width: 50%;
}
#app2 > #tab-bar > li.selected{
    background: palegreen;
    color: white;
}
#app2 > #tab-content > li{
    display: none;
}
#app2 > #tab-content > li.active{
    display: block;
}
#app3{

}
#app3 > .square {
    border: 1px solid rgb(141, 57, 211);
    width: 10vw;
    height: 10vw;
    margin-top: 10vw;
    margin-left: 10vw;
    transition: transform 1s;
}
#app3 > .square:active {
    transform: translateX(15vw);
}
#app4{

}
#app4 > .circle{
    border: 1px solid springgreen;
    width: 15vw;
    height: 15vw;
    border-radius: 50%;
    
}

@keyframes change{
    0%{
        background-color: rgb(15, 219, 32);
    }
    100%{
        background-color: tomato;
    }
}
#app4 > .circle:active{
    animation: change 1s infinite alternate linear ;
}